Block copolymers of conjugated diolefins and vinyl aromatics are useful as elastomeric thermoplastics. The conjugated diolefin segments and the vinyl aromatic segments of these block copolymers are incompatible and phase separate into distinct domains. The continuous domains are rubbery conjugated diolefin domains. When at least some of the conjugated diolefin segments have vinyl aromatic segments at each end, the vinyl aromatic domains will tie together the rubbery segments. At temperatures below the vinyl aromatic domain glass transition temperatures, these copolymers act like vulcanized rubbers. By heating the copolymers to temperatures above the vinyl aromatic domain glass transition temperatures, the copolymers can be melt processed.
The properties of block copolymers can be further improved by hydrogenation of the conjugated diolefin segment. This process greatly increases the ultraviolet, thermal, and oxidative stability of the polymers.
As elastomeric thermoplastics, block copolymers of conjugated diolefin and vinyl aromatics have many excellent qualities, but also have shortcomings. As non-polar polymers, these block copolymers are generally not sufficiently compatible with polar engineering thermoplastics to be useful as tougheners for these engineering thermoplastics. These block copolymers are also easily dissolved in many non-polar solvents. Adhesiveness to polar substrates, and green strength could also be improved. Grafting polar functional groups to the copolymers can help eleviate these shortcomings. U.S. Pat. No. 4,628,072 discloses a method to graft alpha-beta unsaturated carboxylic acids or anhydrides to base block copolymers, and discloses advantages of these functionalized block copolymers as tougheners for various polar engineering thermoplastics. This method of functionalization incorporates the grafted groups mainly into the conjugated diolefin segments. These processes can be performed in a melt phase, and are commonly achieved in an extruder. This results in a very economical process.
Functionalization of the conjugated diolefin segments of the polymer may adversely affect elastomeric properties of the block copolymer. Further, high temperature properties of those block copolymers are generally limited by the glass transition temperature of the vinyl aromatic domains. Grafting some functional groups to vinyl aromatic segments can result in ionic associations within the vinyl aromatic domains which increase the glass transition temperature of the vinyl aromatic domains. This higher glass transition temperature results in significantly improving high temperature mechanical properties. It would therefore be preferable to graft the functional groups within the vinyl aromatic segments.
U.S. Pat. Nos. 3,976,628 and 4,145,298 teach processes to graft polar functionality to vinyl aromatic segments of block copolymers. The processes taught in these patents involve metalating a base polymer with a lithium alkyl in the presence of tetraalkylethylenediamine in an inert solvent and then further reacting the grafted lithium atom with carbon dioxide to form a carboxylic lithium salt. These references teach metalation in inert solvents, and, if pertinent, subsequent functionalization in the same inert solvents. These processes result in selective functionalization of vinyl aromatic blocks, but are performed in solvents. These solution based processes are undesirable because they are more expensive than melt phase processes.
These prior art processes for metalation and further functionalization of hydrogenated copolymers of vinyl aromatics and conjugated diolefins have been performed in inert solvents. Generally the solutions are 5 to 20% weight by polymer. The solvent must be heated to reaction temperatures and separated from the functionalized polymer when the reactions are finished. This adds considerable time and expense to the process. Accomplishing this process in a melt i.e. solvent free, would therefore be preferred.
Some functionalization chemistries are known to be possible in the melt phase. For example, U.S. Pat. No. 5,066,726 discloses a process to graft epoxy containing functional monomers onto a base polymer of the type discussed herein. This is a free radical process which requires the use of a free radical initiator. In free radical chemistry, it is known to use different initiators when operating at different temperatures, i.e., some initiators are good only for lower temperatures and others are good for use at higher temperatures.
In contrast, in metalation chemistry, one has to use the same initiators at high temperatures as are known to be useful in a lower temperature solution process because higher temperature stable alternatives are not available. It is well known that metal alkyls are unstable at higher temperatures. For example, "The Pyrolysis of Unsolvated Alkyl Lithium Compounds" by Glaze et al., Journal of Organic Chemistry, August 1966, pages 2643-2645, discusses how such alkyl lithium compounds including sec-butyl lithium, thermally decompose at temperatures in the range of 82.degree.-100.degree. C. Also, "Organometallic Chemistry. XII. The Thermal Decomposition of n-butyl Lithium, A Kinetic Study" by Finigan et al., Journal of Organic Chemistry, December 1965, pages 4138-4144, discusses the thermal decomposition of n-butyl lithium to butene-1 and lithium hydride at temperatures in the range of 100.degree.-150.degree. C. Finally, "The Rate of Ethylene Polymerization Initiated by Various Chelating Tertiary Diamine: n-butyl Lithium Complexes" by Marshall et al., Journal of Applied Polyer Science, Volume 42, pages 533-541 (1991), discusses the composition of butyl lithium in the presence of a diamine at temperatures within these ranges. Thus, it is apparent in the art that these compounds are unstable at these temperatures and that makes it unlikely that metalation, which requires these compounds, could be carried out in the melt because of the higher temperatures required for melt metalation rather than solution metalation.
There are a number of chemistries which have not worked in melt form on copolymers like those discussed herein. For example, neither sulfonation nor epoxidation of this type of polymer will work in the melt. Also, melt metalation to specifically add functionality to the vinyl aromatic hydrocarbon block is possible according to the present invention. However, with chemistries such as carbene and nitrene, it is not possible to specifically add the functionality only to either the aromatic hydrocarbon block or the conjugated diene block.
It is therefore an object of this invention to provide a process to incorporate functional groups onto vinyl aromatic segments of hydrogenated block copolymers, wherein the process is performed in a melt phase.